The present invention relates to a method and apparatus for evaluating the degree of cure in polymeric composites. More particularly, the invention is directed toward evaluating the degree of cure in carbon-fiber-reinforced plastics, in a rapid and non-destructive manner.
Many products are made from thermosetting resins because fiber-reinforced plastics, lacquers and adhesives based on these resins generally have improved mechanical and chemical properties. They exhibit good mechanical and thermal stability and resist a wide variety of highly reactive chemicals. As with many other polymeric materials, development and control of the manufacturing process require a monitoring of the degree of cure.
This is particularly true with respect to graphite-epoxy composites which are subjected to strict inspection procedures because of their increasing utilization as structural composites in safety related fields such as in the aerospace industry. The mechanical Properties of carbon-fiber-reinforced plastics structures are much affected by the degree of cure of the resin matrix both before and after processing. Before processing, the slightly precured prepreg sheets (i.e. epoxy pre-impregnated carbon fiber sheets) which are shipped to the manufacturer in refrigerated cells are normally inspected to verify that the required pre-cure level has not been exceeded. After lay-up and autoclave curing, the degree of polymerization must be sufficiently high to assure the required mechanical performance.
A number of approaches are possible to evaluate the degree of polymer cure, including spectroscopic, calorimetric, mechanical, electromagnetic or ultrasonic methods, the most widely used being the spectroscopic and the thermal techniques. Infrared spectroscopy can provide quantitative data concerning the amount of unreacted epoxy groups. For best results, measurements must be made in transmission and, in the case of cured composites, this requires destruction (e.g. grinding) of the sample. The spectroscopic diffuse reflectance method has the advantage of being noncontact and nondestructive, but is of limited value for light-absorbing materials such as graphite-epoxy composites.
Thermoanalytical methods include differential thermal analysis, differential scanning calorimetry, thermomechanical analysis and thermogravimetry techniques. The thermoanalytical approach is very powerful but rather time-consuming, requiring careful sample preparation and good thermocouple contact.